464 F. J. H. Merrill — Post- Glacial History 



the plane of erosion approximates to horizontality, and when it 

 becomes infinitely great the plane of erosion will become a 

 base-level. As the cutting rate relatively diminishes, the plane 

 of erosion will become more and more inclined to the base- 

 level and will approach vertically. When the cutting rate 

 becomes infinitely small with respect to the rate of land move- 

 ment the plane of erosion becomes vertical. In this case a 

 vertical rock face would not lose its verticality by the erosion 

 nor would the slope of the land surface be altered except 

 through variations in the resistance of the rock acted upon. 



The degree in which the eroded land surface would approxi- 

 mate to an oblique or vertical plane of erosion would depend 

 upon the previous configuration of that surface. In order to 

 completely discuss this question it would be necessary to con- 

 sider a large number of incidental factors which might divert 

 the plane of erosion from its theoretic position and prevent the 

 eroded land surface from coinciding with it, but this complete- 

 ness is unnecessary for the present purpose which is simply to 

 point out the fact that a land surface in process of subsidence 

 or emergence may be subjected to wave action without being 

 incised with distinct shore lines, and also that wave action may 

 produce an inclined plane as well as a terrace or a oase-leveL 



It is therefore evident that submergence would not leave a 

 deeply cut shore-line as its record unless the rates of land 

 movement and wave cutting were so adjusted as to permit of 

 it. In fact, no very distinctly cut shore lines are to be found 

 on the drift about New York even at an altitude correspond- 

 ing with that of the Hudson estuary deposits. Apart from the 

 still water deposits the 80 foot post-Glacial depression about 

 New York can only be traced by change of surface slope and 

 material at this level. Even these two varieties of evidence 

 are not always co-existent. 



There are in Westchester County and on Long Island indi- 

 cations of wave action on the glacial drift at altitudes of 150 to 

 180 feet, it remains to be determined whether they are reliable. 



The present condition of the Quaternary deposits in the 

 Hudson valley is indicative of fluviatile erosion in post-Cham- 

 plain time. The estuary deposits and deltas have been eroded 

 and truncated until but a narrow fringe is left of formations 

 which once extended far across the valley or filled it, and the 

 water in the channel of the river has now a depth varying 

 from 50 feet in the shallower portions to 180 feet in the 

 deepest parts. 



The delta deposits have also been subjected to the erosion of 

 the streams which formed them and which developed cutting 

 power as the land rose from its submergence. This erosion 

 removed a large portion of the deposits and excavated chan- 



